1. Field of the Invention
This invention relates to an image forming apparatus that forms an image, which is based on image data, on a recording medium that is transferred by a loop belt.
2. Description of Related Art
An image forming apparatus, such as a copier or printer, has loop belts arranged in respective sections, so that a recording sheet is carried properly from a containing section, in which recording sheets are stored, to an ejecting section, to which a recording sheet forming an image based on image data is ejected, using the loop belts. Such a loop belt is, for example, employed for a process unit that forms an electrostatic latent image, which is based on the image data obtained from the outside, on a photosensitive drum and develops the latent image into a developed image, using a developer, and for a transfer unit that transfers the developed image formed on the photosensitive drum to a sheet, and for a fixing unit that fixes the transferred image to the sheet.
Recently, a color image forming apparatus using an intermediate transfer method has been in practice. According to the intermediate transfer method, developed images based on image data of a plurality of hues are formed on photosensitive drums corresponding to respective hues, and the developed images of respective hues are layered on an intermediate transfer body, then the layered developed images having respective hues are transferred all together again from the intermediate transfer body to a sheet. A loop belt is also used in such an application as the intermediate transfer body, and the color image forming apparatus like this enables a faster image forming process.
The loop belt as described above is stretched between a plurality of rollers, including a driving roller, and is driven by the driving roller to run in rotation. The loop belt, therefore, may meander as it is given a specific tensile force in the course of rotation. Since a loop belt used for an image forming apparatus works in a process where positional precision is essential, such as a process of transferring or fixing a developed image on a photosensitive drum to a sheet, preventing the meandering of the loop belt is an absolute necessity.
There is an apparatus that employs one method of preventing the meandering of a loop belt. According to the method, the loop belt has recessions along both edges thereof, where the recessions extend in parallel with the running direction of the loop belt, and rollers, over which the loop belt is stretched, are provided with bias stoppers, such as ribs, on both ends thereof (See Japanese Patent Application Laid-Open No. 2001-96551). By fitting the bias stoppers on the rollers into the recessions of the loop belt as the rollers rotate, the loop belt is prevented from meandering while it runs.
Since a loop belt used for an image forming apparatus runs in rotation in a stretched manner, it must be made of a material showing superior bendability. Besides, the loop belt carries a recording sheet or a developed image transferred to the loop belt from a photosensitive drum, both are held on the loop belt, according to a location where the loop belt is arranged in the image forming apparatus. Because of this, the loop belt is made of material that has an electric resistance according to a given location of arrangement (for example, 1×10 6to 1×10 13Ω cm or so).
Conventionally, the loop belt rotates as many times as the image forming apparatus executes image processes, and is subjected to a number of electric fields generated by voltages that are applied to the loop belt according to a given location. As a result, the bending capability of the loop belt declines due to continuous rotations, and both edges of the loop belt, which are parallel with its running direction, deteriorates heavily as well.
Meanwhile, in forming a static latent image based on image data on a photosensitive drum, an electrifier is used to electrify the surface of the photosensitive drum. A charger-type electrifier is in popular use, because it costs less in comparison with a roller-type and a brush-type. The charger-type electrifier, however, generates more ozone than the roller-type or brush-type does when the charger-type electrifier electrifies the photosensitive drum.
In addition, the loop belt deteriorates not only due to a rotation load applied by itself, but also to damage to a belt material by ozone that is generated by an applied voltage to the photosensitive drum. The damage to the belt material by the ozone concentrates particularly on both edges of the loop belt that are parallel with its running direction. When the ozone damages the material of the stretched loop belt that runs in rotation under a given tense condition, both edges of the loop belt may yield to an applied tensile force to crack. This concern becomes graver when using a charger-type electrifier.
In an image forming apparatus, a sheet is held and carried at a center portion of a loop belt having a width greater than that of the sheet. As the sheet is carried in this manner, the sheet is electrified via the loop belt to transfer a developed image formed on a photosensitive drum to the sheet. In the transfer process, the area occupied by the sheet held on the surface of the loop belt comes in contact with the photosensitive drum via the sheet, but the area of both sides of the occupied sheet comes in direct contact with the photosensitive drum, allowing the voltage corresponding to the charged electric potential of the loop belt to be applied to the photosensitive drum. This means that a high voltage is applied to the area other than an area for forming a static latent image on the photosensitive drum, that is, to the area on the photosensitive drum that corresponds to the outside of the area on the loop belt where the sheet is held and carried. The high voltage applied to such an area on the photosensitive drum causes it to deteriorate.